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need help diagnosing "limp" mode

1993 750iL (prod. 12/1992) increasingly went into limp mode over about a 15 minute span, starting out intermittent and ending up full time limp (low power, runs rough, etc.)

0) Reading the codes with the Peake shows only O2 sensor (for both banks).

1) Disconnecting the right MAF had no effect, meaning it still starts and runs rough. Disconnecting the left MAF and she won't start.

2) Putting a timing light on the right coil wire shows intermittent pulses at idle and no pulses at all above about 1K rpm. Checking the left coil wire shows strong consistent pulses.

3) Both distributor caps, rotors, wires all check good. Both coils have correct resistance on primary and secondary sides.

4) Both crankshaft sensors have correct resistance (540 ohms).

Assuming one bank of the V12 (right side?) is not running, I tried swapping some parts to isolate the issue.
- swapped MAFs and #1 above still holds true.
- swapped throttle bodies and #1 above still holds true.

Sorry, this is NOT limp mode

I know this because if I put the shifter in '3', it starts in 1st gear, then shifts later to 2nd as it accelerates, and finally up to 3rd gear. My understanding is that limp mode starts and stays in 3rd gear.

Found: bad fuel pump #2

After disconnecting the electrical connector to the fuel pumps/sending unit from the trunk, I checked resistance between pin 4 and pin 5 (fuel pump #1) and showed just less than 2 ohms. Checking resistance between pin 4 and pin 6 (fuel pump #2) showed an open circuit!

I'm considering installing a Y-connector where the two fuel feed lines meet the fuel rails so that the one good fuel pump will drive both fuel rails. Than should let me drive the car, as long as I'm not racing it, until both (yes, both!) new fuel pumps arrive. Anyone thinking I'm crazy to try this should let me know why.

Re: no, bad oxyden sensor cannot cause this

Thanks Shogun.

After doing some more research (and even the Bentley manual mentions this) I learned that the O2 sensors are used by the DME for fuel/air mixture only. Anyway, their signals aren't used until the engine warms up.

I also found a post on Roadfly (and Sean and Johan's site confirms it) that states that a faulty fuel pump will NOT set a diagnostic code (through the CCM). This is what really tripped me up, and is definitely a design flaw.

good idea with the Y-connection

I believe one pump will be enough to pump enough fuel as long as you do not drive full throttle.
Original the capacity is laid out per pump for 875 ccm/30 seconds at 3 bar, so theoretical that would be >100 liters per hour!Per pump of course.
You can also try to install these cheaper pumps from TrePerformace, they are claiming to pump our 255 l/h. I have asked someone to take the dimensions.
Of course most of it goes back with the return line.

Just use 1 pump then

That TREperformance fuel pump has more than enough power to push fuel through both fuel lines in the 750iL. I would just use one pump and a Y-connector inside the fuel tank.

Of course, you would lose that nice feature of being able to drive home on one cylinder bank when one of the two fuel pumps goes bad (like I was able to do).

According to their website, the current price of the BMW 255 LPH fuel pump is $69.99. They are right down the street from me! Let me know if you want any information on it, such as dimensions in cm, and I'll go there and measure it.

that would be nice if you can get the data

or after you changed the fuel pumps, compare both in size, the orginal Bosch 69493 with that one.

The Tre-performance pump might be enough with regards to capacity, although that is probably under optimum conditions, maybe without counterpressure. The original Bosch pump has to produce the capacity I mentioned under counterpressure of 3 bar.

Another point to consider is: study the wiring diagrams, what happens if one pump is only installed? Any feedback to the EML or the MOTRONIC?
Can't find it on the MOTRONIC pinout, maybe o.k.
http://www.europeantransmission.com/Bulletin/DTC.BMW/lateversion_zf_pinout88-89.pdfShogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html

1 pump works just fine, thank you

Bought a nice Y-connector for $2.99 at Kragen Auto. Connected the double ends to the fuel rails and connected the single end to the right side fuel line (the one connected to the remaining good fuel pump); all using some short sections of new fuel hose.

Capped off the left side fuel line using an extra vacuum cap from when I replaced the ones on the throttle bodies; fit perfect.

After warming it up I found some open road and did some hard acceleration and she behaved just fine, no lurching or sluggishness.

I won't leave it this way for long. The plan is to get a couple TrePerformance fuel pumps and retrofit them in the plastic holder in the fuel tank. I'll report on the when it's done.

gasoline insulates!

I'm laughing at myself right now, because I just realized why the electrical connectors to the fuel pumps sitting inside the fuel tank immersed in gasoline don't short out. Gasoline, being an oil, is an insulator.

Though I could see if you got a lot of water in the tank and the fuel level was just right, the fuses would pop...

no effect

I've been running with only one fuel pump for a few weeks now and all is well. I removed the fuel pump relay that controls the bad fuel pump which I previously disconnected. There are no CEL codes coming up, no EML warning, nor any other adverse behavior.

I'm going over to TrePerformance today to buy two new fuel pumps and will install them this weekend. I'll post the physical dimensions of their fuel pump as well as my results here and probably on [Oops!] (under username tornacious).

fuel pumps replaced

I just replaced both fuel pumps with after market ones and it worked great. For the details, see my posting in the E32 forum of bimmer board dot com. The thread name is: "750 M70 also runs with one fuel pump"